1. Field of the Invention
The present invention relates to reusable automatic injection devices. In particular, the present invention relates to automatic injection devices including a spring-loaded drive mechanism that incorporates one or more drive springs formed of a shape memory alloy.
2. State of the Art
Automatic injectors (hereinafter referred to as “autoinjectors”) incorporating needled injection mechanisms are well known and are thought to exhibit several advantages relative to simple hypodermic syringes. For instance, because autoinjectors may be designed to automatically and reliably deliver a desired dose of medicament, they facilitate quick, convenient, and accurate delivery of medicaments. In particular, autoinjectors are well suited for use by subjects who must self-administer therapeutic substances or by healthcare workers who must inject multiple subjects over a relatively short period of time. Moreover, autoinjectors incorporating a needled injection mechanism may be designed so that the needle is hidden from view before, during, and even after an injection operation, thereby reducing or eliminating any anxiety associated with the act of penetrating a visible needle into the subject's tissue. Though their precise specifications vary widely, needled autoinjectors generally include a body or housing, a needled syringe or similar device, and one or more drive mechanisms for inserting a needle into the tissue of the subject and delivering a desired dose of liquid medicament through the inserted needle.
The drive mechanisms included in state of the art needled autoinjectors generally include a source of energy capable of powering the drive mechanism. This energy source may be, for example, mechanical (i.e., spring-loaded), pneumatic, electromechanical, or chemical, as described in U.S. Pat. Nos. 6,149,626, 6,099,504, 5,957,897, 5,695,472, 5,665,071, 5,567160, 5,527,287, 5,354,286, 5,300,030, 5,102,393, 5,092,843, 4,894,054, 4,678,461, and 3,797,489, the contents of each such patent being incorporated herein by reference. International Publications numbered WO 01/17593, WO 98/00188, WO 95/29720, WO 95/31235, and WO 94/13342 also describe various injectors including different drive mechanisms. Nevertheless, needled autoinjectors more often incorporate drive mechanisms that utilize a coil spring as an energy source. Such spring-loaded drive mechanisms are desirable because they are thought to facilitate the creation of reliable autoinjectors that are relatively simple in design and inexpensive to manufacture.
In light of the growing desire to deliver increasingly viscous medicaments via a needled injection device, however, known spring-loaded drive mechanisms exhibit significant disadvantages. Specifically, the spring-loaded drive mechanisms included in state of the art needled autoinjectors are typically designed to generate forces sufficient for the injection of low viscosity medicaments, such as insulin and epinephrine, which generally exhibit viscosities near that of water (i.e., about 1 centipoise at 20° C.). Consequently, the spring-loaded drive mechanisms included in known autoinjectors are designed to exert only small injection forces (e.g., ranging from about 1 lb. to about 5 lbs.), which are not suitable for the delivery of emerging, injectable medicaments, such as bioerodible depot formulations, having viscosities much higher than that of water. As can be predicted using the Hagen-Poiseuille Law (F=8QμL(R2/r4)), wherein “F” represents the injection force required, “Q” represents the flow rate of the material injected, “μ” represents the viscosity of the material injected, “L” represents the length of the needle used, “R” represents the internal diameter of the reservoir containing the material to be injected, and “r” represents the internal diameter of the needle used, the injection forces required to deliver a dose of medicament through a needle of desirable gauge will easily exceed those typically provided by state of the art spring-loaded autoinjectors if the viscosity of the medicament to be delivered increases significantly beyond 1 centipoise.
A possible solution to the need for a spring-loaded drive mechanism capable of generating injection forces suitable for delivering higher viscosity medicaments would be to simply provide a drive mechanism including a heavier conventional spring capable of exerting a higher injection force. Yet, such an approach is not without difficulties. In particular, where the injector is designed as a multiple use device, the spring-loaded drive mechanism must be cocked such that the drive spring is held in a compressed position before each use, and in order to cock a conventional spring-loaded drive mechanism, a force that is equal to or greater than the maximum force exerted by the drive spring must be applied to the drive mechanism. It can be appreciated, then, that as the viscosity of the medicament to be delivered increases, not only does the injection force required to deliver the medicament increase, but the force required to cock the drive mechanism also increases. Where the material to be injected exhibits viscosities that approach those of proposed depot materials, the force required to cock a spring driven mechanism designed for delivery of the medicament could exceed that which could be reasonably applied by a user, even if the injector is provided with a cocking mechanism that provides some mechanical advantage that reduces the force that must be directly applied by the user to cock the drive mechanism.
It would be an improvement in the art, therefore, to provide a multiple use, spring-loaded autoinjector that includes a drive mechanism that can be cocked by a force that is lower than the injection force provided by the drive mechanism. Such an autoinjector could be designed to provide an injection force that is higher than the injection forces typically exerted by state of the art spring-loaded autoinjectors, while still allowing the user to cock the drive mechanism for reuse through the application of a force that is practically applicable.